1. Field of the Invention
The present invention is directed generally configuration of a cellular wireless network, and more particularly, to an apparatus and method of assigning configuration parameters to transceivers in a cellular wireless network.
2. Description of the Related Art
A wireless network includes a plurality of base stations (“BS”) each having one or more transceivers configured to transmit and receive signals over at least one wireless communication link. The area covered by a BS is typically divided into sectors, each sector having one or more transceiver. A mobile station (“MS”) transmits data to a BS on an uplink (“UL”) portion of the communication link and receives data from the BS on a downlink (“DL”) portion of the same communication link.
Worldwide Interoperability for Microwave Access (“WiMAX”) broadband services are provided over channels, which are typically 5 MHz, 10 MHz, or 20 MHz. A different channel is typically assigned to each sector of a BS. If multiple transceivers are operating within a sector, each transceiver may be assigned a different channel. However, within the wireless network, channels are typically reused in well known reuse schemes (such as a conventional WiMAX 1/3/3 reuse scheme and the like).
At the physical layer of the Open Systems Interconnection Reference Model (or OSI Model), WiMAX uses orthogonal frequency-division multiplexing (“OFDM”) to divide a channel into a large number of closely-spaced orthogonal subcarriers. To provide multiple access, WiMAX uses Orthogonal Frequency-Division Multiple Access (“OFDMA”) to assign subsets of subcarriers to sub-channels. Each sub-channel is then assigned to an individual user.
Sub-channelization refers to the division of available subcarriers into sub-channels. Sub-channels may include contiguous subcarriers or subcarriers pseudo-randomly distributed across the frequency spectrum. WiMAX defines several sub-channelization schemes based on distributed carriers for both the UL and the DL. For example, partial usage of subcarriers (“PUSC”) and full usage of the subcarriers (“FUSC”) are sub-channelization schemes using subcarriers distributed across the frequency spectrum.
A WiMAX MS transmits UL and DL data signals over the sub-channel assigned to a user in frames. Each frame includes an UL portion and a DL portion. The frame may also include a DL map (“DL_MAP”) portion that includes information about how the DL portion is structured. The MS may use the DL_MAP to decode the DL portion of the frame.
The total communication capacity of the frame may be viewed as a time/frequency grid or symbol/sub-channel grid. The time/frequency grid of the frame may be divided into permutation zones, which are groupings of contiguous symbols that use the same sub-channelization scheme. Thus, the frame may include a DL PUSC permutation zone, a DL FUSC permutation zone, and the like. Each of the permutation zones is divided into slots, a slot being the basic unit of allocation in the symbol/sub-channel grid.
In implementations in which broadband services are provided over a 10 MHz channel and a 1024-FFT OFDMA symbol is used, there are typically 840 pilot and data subcarriers assigned to the DL PUSC zone. According to applicable WiMAX standards (e.g., WiMAX air-interface standard Institute of Electrical and Electronics Engineers (“IEEE”) 802.16e-2005), the first step in sub-channelizing the DL PUSC zone is to divide the 840 subcarriers into 60 clusters each having 14 adjacent subcarriers (60*14=840).
Next, these clusters are renumbered or assigned logical numbers (“LN”) using a first predefined formula. The first predefined formula includes a Downlink Permutation (“DL_PermBase”) index value. The DL_PermBase index value is an integer within the range of 0 to 31. The DL_PermBase index value is assigned to the transceiver that transmits the DL portion of the frame and may be transmitted thereby to the MS in the DL_MAP portion of WiMAX frames.
Then, the clusters are each assigned to one of six groups based on their LN. Lastly, the subcarriers are assigned to sub-channels. In this example, the frame includes 30 sub-channels. Each sub-channel includes 24 subcarriers to be used as data carriers and 4 subcarriers to be used as pilot carriers. Thus, the frame includes 840 (30*(24+4)=840) data carriers. In each sub-channel, the subcarriers belong to the same group. A second predefined formula, including the DL_PermBase index value, is used to assign the subcarriers to the sub-channels.
The DL FUSC zone is sub-channelized by dividing the subcarriers into groups of contiguous subcarriers. If the channel is 10 MHz, the subcarriers are divided into 48 groups, numbered 0 to 47. Then, each sub-channel is constructed by assigning a subcarrier from each group to the sub-channel. Thus, 16 sub-channels each having 48 subcarriers are created for a 10 MHz channel.
Partial usage of sub-channels (“PUSC”) and full usage of sub-channels (“FUSC”) may be characterized as distributed permutations in which the subcarriers are assigned to the sub-channels pseudo-randomly. The main advantages of such distributed permutations are frequency diversity and inter-cell interference averaging. Diversity permutations minimize the probability of using the same subcarrier in nearby sectors or cells. On the other hand, channel estimation may be difficult because the subcarriers are distributed over the available bandwidth of the channel.
As explained above, the DL_PermBase index value dictates at least in part the assignment of subcarriers to sub-channels in the DL PUSC zone. This process may help reduce interference in the DL. For example, if two nearby BS sectors are assigned the same channel and different DL_PermBase indices, the interference level between the nearby sectors will be lower than if the nearby sectors were assigned the same channel and the same DL_PermBase index value. In the later case, the interference between the nearby sectors would be 100%.
Currently, DL_PermBase index values are assigned manually to each transceiver in the wireless network by a technician. The assignment is fixed per the technician's initial assignment and does not change during operation of the wireless network. Therefore, a need exists for a technique to assign DL_PermBase index values to transceivers in a wireless network that considers actual operating conditions of the wireless network. A technique that considers actual interference between nearby transceivers would be particularly beneficial. The present application provides these and other advantages as will be apparent from the following detailed description and accompanying figures.